Method for depositing an amorphous silicon film for photovoltaic devices with reduced light-induced degradation for improved stabilized performance

ABSTRACT

A thin film photovoltaic device on a substrate is being realized by a method for manufacturing a p-i-n junction semiconductor layer stack with a p-type microcrystalline silicon layer, a p-type amorphous silicon layer, a buffer silicon layer comprising preferably intrinsic amorphous silicon, an intrinsic type amorphous silicon layer, and an n-type silicon layer over the intrinsic type amorphous silicon layer.

FIELD OF THE INVENTION

The present invention relates to improvements in the overallmanufacturing process for thin-film, silicon-based solar cells ormodules relying on the so called amorphous multiple- or single-junctionstructures.

BACKGROUND ART

Photovoltaic devices, photoelectric conversion devices or solar cellsare devices which convert light, especially sunlight into direct current(DC) electrical power. For low-cost mass production thin film solarcells are being of interest since they allow using glass, glass ceramicsor other rigid or flexible substrates as a base material (substrate)instead of crystalline or polycrystalline silicon. The solar cellstructure, i. e. the layer sequence responsible for or capable of thephotovoltaic effect is being deposited in thin layers. This depositionmay take place under atmospheric or vacuum conditions. Depositiontechniques are widely known in the art, such as PVD, CVD, PECVD, APCVD,. . . all being used in semiconductor technology.

A thin-film solar cell generally includes a first electrode, one or moresemiconductor thin-film p-i-n or n-i-p junctions, and a secondelectrode, which are successively stacked on a substrate. Each p-i-njunction or thin-film photoelectric conversion unit includes an i-typelayer sandwiched between a p-type layer and an n-type layer(p-type=positively doped, n-type=negatively doped). The i-type layer,which is a substantially intrinsic semiconductor layer, occupies themost part of the thickness of the thin-film p-i-n junction.Photoelectric conversion occurs primarily in this i-type layer.

Prior Art FIG. 1 shows a basic, simple photovoltaic cell 40 comprising atransparent substrate 41, e. g. glass with a layer of a transparentconductive oxide (TCO) 42 deposited thereon. This layer is also calledfront contact F/C and acts as first electrode for the photovoltaicelement. The next layer 43 acts as the active photovoltaic layer andcomprises three “sub-layers” forming a p-i-n junction. Said layer 43comprises hydrogenated microcrystalline, nanocrystalline or amorphoussilicon or a combination thereof. Sublayer 44 (adjacent to TCO frontcontact 42) is positively doped, the adjacent sub-layer 45 is intrinsic,and the final sub-layer 46 is negatively doped. In an alternativeembodiment the layer sequence p-i-n as described can be inverted ton-i-p, then layer 44 is identified as n-layer, layer 45 again asintrinsic, layer 46 as p-layer. Finally, the cell includes a rearcontact layer 47 (also called back contact, B/C) which may be made ofzinc oxide, tin oxide or ITO and a reflective layer 48. Alternatively ametallic back contact may be realized, which can combine the physicalproperties of back reflector 48 and back contact 47. For illustrativepurposes, arrows indicate impinging light.

Depending on the crystallinity of the i-type layer solar cells orphotoelectric (conversion) devices are characterized as amorphous (a-Si)or microcrystalline (pc-Si) solar cells, independent of the kind ofcrystallinity of the adjacent p and n-layers. Microcrystalline layersare being understood, as common in the art, as layers comprising atleast a Raman crystallinity of 15% of microcrystalline crystallites inan amorphous matrix.

Nowadays, efficiency of solar cells and low cost production are ofincreasing interest. Multiple-junction solar cells with at least twothin-film photoelectric conversion units stacked one on the other arehighly efficient; however need increased effort in manufacturingequipment. Moreover, the deposition rate for microcrystalline siliconwith PECVD methods in tandem (double-junction) solar cells isconsiderably lower than the rate of amorphous layers (under otherwisecomparable conditions). Therefore, there is a need for low-cost highefficient amorphous silicon solar cells.

Today, thin film solar cells are heading for mass-production.Requirement for such mass production are integrated manufacturingprocesses, allowing efficiently and effectively manufacturing suchcells. Yield, throughput, uptime, quality are ingredients to observe insuch processes. On the other hand, a clear goal is to increase cellefficiency and other electrical properties of the solar cells. However,the so called Stabler-Wronski effect describes the tendency of amorphoussilicon photovoltaic devices to degrade (drop) in their electricityconversion efficiency upon initial exposure to light. Light-soakingexperiments which are internationally accepted, include exposing thetest devices under AM1.5-like illumination at a controlled temperatureof 50° C. during 1000 hrs. The stabilized performances are evaluated atstandard conditions (AM1.5 1000 W/m², 25° C.) and the relativeefficiency degradation is given by the difference between the initial(before light-soaking exposure) and the degraded efficiency (after 1000hrs) normalized to the initial efficiency. Therefore efficiency is notonly important as an initial value but also as a stabilized value.Values of about 20% or more for the relative degradation as known in theart for a single-junction amorphous silicon cell with an i-layerthickness larger than 200 nm are a serious obstacle for thecommercialisation of such PV cells.

SUMMARY OF THE INVENTION

This invention relates to a process for the manufacturing of thin filmphotovoltaic devices that can, for example, be used in architecturalapplications. While this invention is not limited to a specific type ofthin film photovoltaic device, this invention is particularly suitablefor the production of thin film photovoltaic devices comprising anamorphous cell structure with an a-Si intrinsic layer thickness largerthan 200 nm, that display a relative efficiency degradation below 20%and even, for one embodiment of the invention, below 16.5%.

Generally, a thin film photovoltaic device comprises a substrate,preferably a transparent vitreous substrate, usually with a thickness of0.4 mm to 5 mm, preferably 2 mm to 4 mm, an electrically conductivecontact on the substrate, one or more semiconductor layers whichgenerate an electric charge separation upon exposure to light, and asecond electrically conductive contact. The semiconductor layer orlayers are positioned between the electrically conductive contacts. Thesemiconductor layers are deposited in a manner that provides for ajunction; preferably the photovoltaic devices according to thisinvention exhibit at least one p-i-n junction or at least one n-i-pjunction although other types of semiconductor junctions can beutilized. The p-i-n junction can be realized in a semiconductorcomprising p-, i- and n-regions or layers. The i-region is an intrinsicregion, the p-region is typically a positively doped region, and then-region is typically a negatively doped region. Region, in the contextof this invention means comprising at least one layer with at least onecommon property.

The i-region is positioned between the p- and n-regions in the p-i-njunction or the n-i-p junction. It is generally understood that whenlight, for example, solar radiation, impinges on a photoelectric devicecontaining a p-i-n or n-i-p junction, electron-hole pairs are generatedin the i-region. The holes from the generated pair are directed towardsthe p-region and the electrons towards the n-region. The contacts aregenerally directly or indirectly in contact with the p- and n-regions.Current will flow through an external circuit connecting these contactsas long as light continues to impinge on the photoelectric devicethereby generating electron-hole pairs.

According to a process of this invention the substrate used to formphotovoltaic devices can be any suitable substrate for receiving theelectrically conductive contact and semiconductor layers of thephotovoltaic device. The substrate is generally flat and can be glass,glass-ceramics, ceramics or other glass-like material, a plastic such asa polyimide, or a metal film such as aluminum, steel, titanium,chromium, iron, and the like. Glass, particularly a highly transparentor transmissive glass is preferred. The substrate can be in any size andshape, provided it can fit into the processing equipment used in theprocess of this invention. If larger substrate sizes are desired, theprocessing equipment as mentioned herein will need to be sizedaccordingly. However, in order to meet the goal of efficiently producingsolar modules, standardization is desirable.

One size common in the market today is a 1.4 m² glass substrate with 1.1m×1.3 m. This invention however is not limited to this size and may besuccessfully applied to other sizes and shapes, be it either rectangularor square.

DETAILED DESCRIPTION OF THE INVENTION

The inventive process as described in more detail below results in anamorphous pin cell deposited on a front TCO of high stabilizedefficiency, including multiple junction device structures where such anamorphous pin cell is part of.

Following a structure as shown in FIG. 1, on a glass substrate 41 a TCOlayer 42 made from ZnO by means of LPCVD has been deposited. However,other layers and other deposition methods may be used, e. g. SnO₂ asdescribed above. In one embodiment a ZnO TCO layer with 1.8 μmthickness, resistivity: 1.6 10⁻³ ohmscm, more than 10% haze and a totaltransmission with index matching (400 to 900 nm) of above 80% have beendeposited with a TCO 1200 LPCVD system, commercially available fromOerlikon Solar.

A subsequent p-i-n junction semiconductor layer stack 43 was formed froma-Si:H (hydrogenated amorphous silicon)or microcrystalline silicon orboth. FIG. 2 shows details of one possible embodiment of the invention,where a p and a n region each comprising (sub)layers with comparabledoping have been used. Layer (1) and (2) according to FIG. 2 establish ap region, comprising a microcrystalline and an amorphous sublayer. Layer(3) is a buffer layer of amorphous silicon deposited with high hydrogenflux. Layer (4) is an intrinsic a-Si:H layer deposited at more than 3Å/s (5) and (6) establish the n region of said p-i-n junction.

Between the deposition steps resulting in layer (2) and (3) a flush withwater vapor of more than 100 sccm for more than 30 sec at 0.2 mbarfollowed by a pumping of more than 30 sec was performed following theteachings of U.S. Pat. No. 7,344,909.

Finally a further TCO layer as back contact 47 has been realized and aback reflector 48 has been added in order to send back into the lightabsorbing i-layer the initially not trapped light. Both such processesand layers are well known in the art. This cell achieved a stabilizedefficiency above 9% compared to an initial value of 10.9% resulting in arelative degradation of 16.5%.

The embodiment described in FIG. 2 is exemplary for the resultsdescribed. However, the processing temperature can be varied between 180and 250° C. without compromising a stabilized efficiency of 9%. Afrequency between 13.56 MHz and 82 MHz (harmonics of 13.56 MHz) can besuccessfully employed. Basically the ratios between SiH₄, H₂ and dopants(if any)CH₄, TMB, PH₃ are crucial and can be easily derived from FIG. 2.The Power applied to the process chamber will influence the desireddeposition rate but will also influence the crystallinity of the layerand its stability. Since the cells in this example had the size of 1cm², the respective power density per cm² can be easily derived fromFIG. 2.

The manufacturing process for the amorphous silicon layers was performedon a KAI PECVD deposition system, as commercially available fromOerlikon Solar. The ZnO layer was deposited on a known as TCO 1200, alsofrom Oerlikon Solar.

The invention claimed is:
 1. A method of forming a thin filmphotovoltaic device on a substrate with an electrically conductivecontact, comprising: forming a p-i-n junction semiconductor layer stackcomprising: forming a p-type microcrystalline silicon layer on saidelectrically conductive contact; forming a p-type amorphous siliconlayer on said p-type microcrystalline silicon layer; forming anintrinsic type amorphous buffer silicon layer with high hydrogen flux onsaid p-type amorphous silicon layer and increasing an atomic percentageof hydrogen in said intrinsic type amorphous buffer silicon layerrelative to said p-type amorphous silicon layer; forming an intrinsictype amorphous silicon layer on said intrinsic type amorphous buffersilicon layer; and forming an n-type silicon layer over the intrinsictype amorphous silicon layer.
 2. A method according to claim 1, whereinthe silicon layers comprise hydrogenated silicon generated from a gasmixture comprising at least silane and hydrogen.
 3. A method accordingto claim 2, wherein the p-type amorphous silicon layer and the buffersilicon layer are deposited in a gas mixture further comprising methane.4. A method according to claim 1 or 2, wherein the p-type amorphoussilicon layer and the p-type microcrystalline silicon layer aredeposited in a gas mixture comprising boron.
 5. A method according toclaim 3, wherein a ratio of silane flux in sccm to the hydrogen flux insccm during deposition of the buffer silicon layer is substantially10:94, and said ratio during deposition of the intrinsic type amorphoussilicon layer is substantially 10:9.
 6. A method according to claim 3wherein a ratio of silane flux in sccm to the hydrogen flux in sccm tomethane flux in sccm during deposition of the p-type amorphous siliconlayer is substantially 10:18:19.
 7. A method according to claim 5,wherein a ratio of the silane flux in sccm to the hydrogen flux in sccmto methane flux in sccm during deposition of the buffer silicon layer issubstantially 10:94:2.
 8. A method according to claim 1, wherein formingthe n-type silicon layer comprises forming an n-type amorphous siliconlayer; and forming an n-type microcrystalline silicon layer.
 9. Asilicon based thin film photovoltaic device comprising, on a substrate,an electrically conductive contact and subsequently a p-i-n junctionsemiconductor layer stack with: a p-type microcrystalline silicon layer;a p-type amorphous silicon layer on said p-type microcrystalline siliconlayer; an intrinsic type amorphous buffer silicon layer on said p-typeamorphous silicon layer, an atomic percentage of hydrogen in saidintrinsic type amorphous buffer silicon layer being increased relativeto said p-type amorphous silicon layer; an intrinsic type amorphoussilicon layer on said intrinsic type amorphous buffer silicon layer; ann-type silicon layer over the intrinsic type amorphous silicon layer.10. A photovoltaic device according to claim 9 wherein the n-typesilicon layer comprises an n-type amorphous silicon layer and an n-typemicrocrystalline silicon layer.
 11. A photovoltaic device according toclaim 9 or 10, wherein the buffer silicon layer comprises an intrinsictype hydrogenated amorphous silicon layer doped with carbon.
 12. Amethod according to claim 1, further comprising: elevating the atomicpercentage of hydrogen in said intrinsic type amorphous buffer siliconlayer relative to said p-type amorphous silicon layer by forming saidintrinsic type amorphous buffer silicon layer with a hydrogen-to-silaneflow ratio that exceeds ten times the hydrogen-to-silane flow ratio forforming said p-type amorphous silicon layer.
 13. A method according toclaim 12, further comprising: elevating the atomic percentage ofhydrogen in said intrinsic type amorphous buffer silicon layer relativeto said p-type amorphous silicon layer by forming said intrinsic typeamorphous buffer silicon layer with a hydrogen-to-silane flow ratio thatexceeds ten times the hydrogen-to-silane flow ratio for forming saidintrinsic type amorphous silicon layer.